Several of the features and services of both wireline and wireless telephones are based on the location of the telephone. For example, a public safety ("911") system directs aid to the caller at the location of the calling telephone. Such systems function without the caller having to identify the location (an important feature for a person unfamiliar with the area or unable to speak to the public safety personnel). The location of a wireline telephone in relation to the wireline network is static; the location of the telephone is simply the point where the telephone is connected to the network. In contrast, a mobile station by definition has a dynamic relationship with locations in its wireless network; therefore, determining its position is more difficult.
In order to locate a mobile station with a high degree of accuracy, measurements of the distances of the mobile station from at least three fixed points are needed. The distance measurements are used in a triangulation algorithm to calculate a position relative to the fixed points. In wireless networks based on the Global System for Mobile communication (GSM) standard, obtaining the distance measurement to make a location determination is relatively straightforward.
In a GSM wireless network, a mobile station's location is derived from the time of arrival of the mobile station's radio signal at different base transceiver stations (BTS's). Since the locations of the BTS's are known, the location of the mobile station is calculated to a relatively high degree of accuracy. The timing values are readily available because of the manner in which the GSM standard specifies multiplexing of mobile stations on a given radio frequency.
A GSM-based mobile station shares a radio frequency with other mobile stations by broadcasting only during a time slot assigned to it by its serving BTS. However, even if the mobile station broadcasts precisely during its time slot, the BTS does not always receive the mobile station's transmission during the time slot because of propagation delays, which are due to the distance the signal travels. To compensate for the propagation delay, the BTS subtracts the time of arrival from the time offset of the time slot to derive a timing advance (TA) value. The TA value is sent to the mobile station, which "advances" its transmission by that amount, so that the transmission arrives at the BTS during the mobile station's time slot. Given that the speed of radio signal propagation is known (the speed of light), the TA value times the speed of light is the distance between the mobile station and the BTS.
The TA value from the serving BTS is always known, because that is the TA that the mobile station is currently using. In order to determine the location of the mobile station, at least two more TA values from two neighboring BTS's need to be acquired. In the current art, these TA values are acquired by performing "positioning handovers."
According to the current method for performing a positioning handover, the mobile station sends a handover access message to a second BTS. The second BTS calculates the TA value but does not respond to the mobile station. Such lack of response simulates a failed handover and the mobile station takes action as in a normal handover failure; that is, it returns to the previous channel on the original BTS after a predefined time period. In GSM, the predefined time period is 320 ms for a traffic channel supporting voice or data, and 675 ms for a stand-alone dedicated control channel supporting short message service. A second positioning handover is then performed to a different BTS, and so forth, in order to obtain an optimal number of TA values.
The collection of TA information using this method degrades call quality. For each location determination, at least two positioning handovers are performed. Consequently, the user of the mobile station experiences at least two perceptible speech interruptions, or perceptible delays in receiving messages. Furthermore, because the current positioning handover method simulates failed handovers, there is always a risk that the mobile station will not return to the previous channel successfully and thus the call is dropped. Therefore, there is a problem in the art in collecting timing advance values for use in determining the location of a mobile station without disrupting an extant call.